Video signal receivers such as direct broadcast satellite receivers and video cassette recorders (VCRs) provide video output signals as Radio Frequency (RF) modulated signals to a television receiver, for example. The carrier frequency selected for this RF modulation in the United States, for example, is usually the broadcast channel 3 or channel 4 cable television (CATV) carrier frequency. Selection of one of these channels is typically accomplished by means of a user operated manual switch. This switch is usually located on the VCR or satellite receiver unit. The operator using a satellite receiver, for example, is typically directed by an instruction manual to select the broadcast channel (3 or 4) that is not used in the operator's local broadcast area. The operator then selects that same channel on the television receiver to ensure that the television receiver is tuned to the same carrier frequency as the satellite receiver RF output.
A problem may arise when both channel 3 and channel 4 are unavailable for use as modulator channels by the satellite receiver. This may occur if one channel is in use in a particular local broadcast area, and the other channel is unusable/unavailable due to being corrupted by broadcast interference. In another situation, both channels 3 and 4 may already be in use. This may occur, for example, if the output of the satellite receiver is being combined with a cable television signal. In such case one channel may carry a television signal and the other channel may already be used as a modulator channel. In these situations the operator may be forced to use a television signal broadcast channel as the satellite receiver output modulator channel. As a result, the operator loses a television signal broadcast channel since any television signal being transmitted on that channel prior to modulation will be lost in the modulation process.
To address these situations, some advanced video receivers use continuously adjustable RF modulators instead of channel 3/4 switches. The adjustable RF modulators allow an unused television channel in the channel 1-125 band, for example, to be used as the modulator channel. Although the satellite receiver may be manufacturer pre-set to a certain modulator channel, for example channel 65, when delivered to the consumer/operator, the operator is able to select any of the available channels as the modulator channel. The operator may do this by means of a particular menu selection using a remote control device.
Operator adjustable modulators may present problems. One problem that may arise is incompatibility between a video receiver (satellite receiver or VCR) modulator output channel and the channel to which a television receiver, receiving the modulator output, is tuned. Such an incompatibility may occur if the operator is unaware of the modulator channel setting of the video receiver, and sets the television receiver to a different channel. Then, the television receiver tuner will not be set to the same carrier frequency as the video receiver RF output signal. This situation may occur if the operator forgets which modulator channel he has selected, if he inadvertently changes the modulator channel, or the modulator channel is changed without his knowledge. These situations are more likely to arise with video receivers that use operator adjustable modulators with a wide, continuously adjustable frequency selection range. In such a case there is a greater opportunity for an operator to erroneously set a modulator channel, or to forget a previously selected modulator channel. The operator may then set the television receiver to an incompatible channel. As a result the television tuner will not be set to the carrier frequency of the video receiver modulator, and the television receiver will not display a valid picture. The operator may interpret this result as an equipment failure.